Lithium-ion cells and batteries are secondary (i.e., rechargeable) energy storage devices useful as portable power sources for many important applications including cellular phones, portable computers, camcorders, and electric vehicles. One such lithium-ion cell comprises essentially a carbonaceous anode, a lithium-retentive cathode, and a non-aqueous, lithium-ion-conducting electrolyte therebetween. The carbon anode comprises any of the various types of carbon (e.g., graphite, coke, carbon fiber, etc.) which are capable of reversibly storing lithium species, and which are bonded to an electrically conductive current collector (e.g. copper foil) by means of a suitable organic binder (e.g., polyvinyllidene difluoride, PVdF). The cathode comprises such materials as transition metal chalcogenides (e.g., LiCoO.sub.2) or electronically conductive polymers (e.g., polyaniline, polythiophene and their derivatives) which are bonded to an electrically conductive current collector (e.g., aluminum foil) by a suitable organic binder.
Carbon anodes and transition metal chalcogenide cathodes reversibly store lithium species by an insertion mechanism wherein lithium species become retained within the lattices of the carbon and chalcogenide materials. In the carbon anode, charge neutralization occurs between the lithium ions and the .pi. bonds of the carbon, whereas in the metal chalcogenide cathode, charge transfer takes place between the lithium species and the transition metal component of the metal chalcogenide. Chalcogenide compounds include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese with nickel and cobalt oxides being among the more popular cathode materials used commercially. Manganese oxide has been proposed as a low cost alternative to the nickel and cobalt oxides.
The electrolyte in such lithium-ion cell comprises a lithium salt dissolved in a non-aqueous solvent which may be (1) completely liquid, (2) an immobilized liquid, (e.g., gelled or entrapped in a polymer matrix), or (3) a pure polymer. Known polymer matrices for entrapping the electrolyte include polyacrylates, polyurethanes, polydialkylsiloxanes, polymethacrylates, polyphosphazenes, polyethers, and polycarbonates, and may be polymerized in situ in the presence of the electrolyte to trap the electrolyte therein as the polymerization occurs. Known polymers for pure polymer electrolyte systems include polyethylene oxide (PEO), polymethylenepolyethylene oxide (MPEO), or polyphosphazenes (PPE). Known lithium salts for this purpose include, for example, LiPF.sub.6, LiClO.sub.4, LiSCN, LiAlCl.sub.4, LiBF.sub.4, LiN(CF.sub.3 SO.sub.2).sub.2, LiCF.sub.3 SO.sub.3, LiC(SO.sub.2 CF.sub.3).sub.3, LiO.sub.3 SCF.sub.2 CF.sub.3, LiC.sub.6 F.sub.5 SO.sub.3, and LiO.sub.2 CF.sub.3, LiAsF.sub.6, and LiSbF.sub.6. Known organic solvents for the lithium salts include, for example, alkylcarbonates (e.g., propylene carbonate, ethylene carbonate), dialkyl carbonates, cyclic ethers, cyclic esters, glymes, lactones, formates, esters, sulfones, nitrites, and oxazolidinones.
Because of several problems associated with the intial start-up (i.e., formation) of the lithium-ion cell, excess lithium ions (species) are generally preloaded in the cell components, typically at the cathode side prior to assembling the cell. A common approach to manufacturing lithium-ion cells having carbon anodes is to couple excess lithium-retentive cathode material before assembling the cell. Carbon-anode, lithium-ion cells assembled from lithium-retentive cathodes which have been preloaded with lithium are hereinafter referred to as "cathode-loaded" cells.
The first problem, in cathode-loaded cells is inefficiency caused by the loss of useful capacity (i.e., as measured by the amount of lithium pre-loaded into the cathode) during the first charge-discharge cycle of the cell in that it is otherwise available for subsequent reversible interaction with the electrodes. In order to compensate for the amount of lithium expected to be lost (i.e., rendered irreversible) in the first cycle, a common practice in the art is to provide excess lithium-retentive cathode materials. Such irreversible capacity loss is usually referred hereinafter as ICL. This, of course, results in a cell having electrodes which are stoichiometrically unbalanced, as far as their relative reversible lithium storing capability is concerned. Such excess cathode material adds to the size, weight and cost of the cell and correspondingly reduces its energy and power density.
The second problem, associated with the cathode-loaded/carbon cells is gas production occuring during the first cycle of the battery attributable to decomposition of the electrolyte's solvent. Such gassing not only produces a combustible gas but can cause thermal and/or mechanical stress in full assembled cells, swelling of sealed cells, separation of the active material from its metal substrate current collector and depletion and contamination (i.e., by reaction byproducts) of the electrolyte. All of which contribute to increases to the cells internal resistance.
The third problem in cathode-loaded cells deals with the depletion of useful, pre-determined amounts of ionically-conducting electrolyte. The problem is especially severe in the cases with immobilized electrolytes or polymer electrolytes or "dry" cells. The cells are typically assembled with electrolyte-unsaturated carbon anodes which only become fully wetted with electrolyte after the cell's first cycle as electrolyte being carried from the electrolyte region of the cell into the carbon by the migrating lithium ions. Such movement of electrolyte out of the electrolyte region and into the carbon anode is particularly troublesome in immobilized-electrolyte cells, since the electrolyte region is depleted of some of its electrolyte and accordingly results in increased internal resistance within the cell.
The aforesaid problems are particularly troublesome in larger batteries such as might be used to propel an electric or hybrid-electric vehicle. The first and second of these problems are seen to be attributed at least in part to the presence of "active sites" throughout the carbon particles. Active sites are defect, surface and edge sites which are characterized by unsaturated interatomic bonds that are prevalent on the ends of the carbon chains and at stacking faults or cracks in the carbon. Such sites have energy potentials above the electrodepostion potential of lithium. High concentrations of active sites are particularly prevalent in fully high surface area carbonaceous materials, particularly in those not exposed to high heat treatment (T&lt;2000.degree. C.) and/or less ordered (i.e. graphitized) carbon. Active carbon sites are troublesome in cathode-loaded cells because they are seen to promote (1) decomposition of the electrolyte's solvent, (2) gassing within the assembled cell incident to such decomposition, and (3) the non-useful consumption of some of the preloaded lithium thereby rendering such lithium no longer available for reversible interaction between the electrodes. Moreover, the extent of electrolyte decomposition is sensitive to the surface area of the carbon. Smaller carbon particles (usually having higher surface areas) tend to have more active sites, and accordingly result in more decomposition (e.g., gassing) of the pre-lithiation solution.
It is generally accepted that during the first cycle of a rechargeable lithium-ion cell, the surface of the electrodes (particularly the carbonaceous anode) become covered by a passivating layer or film, often called a solid electrolyte interphase (SEI), containing a relatively complex ratio of several passivating film components, such as lithium-containing compositions (e.g., Li.sub.2 CO.sub.3, Li.sub.2 O, and Li-alkyl CO.sub.3). The SEI plays a major role in determining electrode and battery behavior and properties which include ICL, lithium deposition-dissolution efficiency, cycle life, and the like. The role of the SEI is to separate the negative electrodes from the electrolytes, to eliminate (or drastically reduce) the transfer of electrons from the electrodes to the electrolytes and also the transfer of solvent molecules and salt anions from the electrolytes to the electrodes which prevents further electrolyte decomposition and gas formation. Heretofore, the compositions or components of the SEI on the electrode, and their ratio to each other, have been dependent entirely upon and determined by the compositions of the electrolytes and electrodes of the particular lithium-ion electrochemical cell in which the electrode is first cycled. The SEI or passivating layer on a "prewet" electrode produced from the first cycle of an electrochemical cell contains an "electrochemically-produced ratio" of the passivating film components. U.S. Pat. No. 5,743,921, issued to Nazri et al., describes the use of a prewet electrode. In view of the importance of the role of the SEI for reducing ICL and other performance issues, a need exists for an improved carbonaceous anode, its preparation and use in cathode-loaded, lithium-ion rechargeable batteries and cells.